Optical recording medium

ABSTRACT

An optical recording medium is provided, which has reduced degradation in jitter caused by manufacturing variations of a semiconductor laser emitting laser beams or variations in output power of laser beams, and which dissipates heat in the recording layer during recording by laser beams to increase the power margin of the laser beam providing playback jitter values at a certain level or less.  
     An optical recording medium  10  has a support substrate  12 , on which formed are a reflective film  16 , a second dielectric layer  18 , a recording layer  20 , a first dielectric layer  22 , a heat sink layer  24 , and a light-transmitting layer  26 . The heat sink layer  24  is formed of a material having a thermal conductivity within a certain range, e.g., alumina, to dissipate heat through the heat sink layer  24  when the heat is generated by a laser beam incident from the light-transmitting layer  26 , thereby preventing an increase in temperature of the recording layer  20.

TECHNICAL FIELD

[0001] The present invention relates to an optical recording medium, andmore particularly to an optical recording medium which provides a widepower margin.

BACKGROUND ART

[0002] As conventional recording media for recording digital data,optical recording media (discs) represented by CDs (Compact Discs) andDVDs (Digital Versatile Discs) have been widely used. The opticalrecording media have widely employed a method of recording data in whichdata to be recorded is modulated into a length of recording marks alongtracks. Additionally, these optical recording media are fabricated suchthat their various as-fabricated (initial) characteristics (electricaland mechanical properties) comply with predetermined specifications, andin particular, their playback jitter values are required to be equal toor less than a certain value as a fundamental property.

[0003] One of the factors responsible for variations in playback jittervalue is a variation in the power of a laser beam during recordingoperations.

[0004] An insufficient amount of power of the laser beam being suppliedduring recording operations would not accurately form marks ofpredetermined lengths, thereby causing an increase in playback jittervalue. On the other hand, an excessive amount of power of the laser beamwould cause an increase in jitter value due to deformation or the likein recording marks.

[0005] This will be explained below in more detail. To read data whenthe aforementioned recording method is employed, an optical recordingmedium is irradiated with a laser beam along the tacks to detect thereflected light, thereby reading information carried by the recordingmarks. On the other hand, to record data, the optical recording mediumis irradiated with a laser beam along the tracks to form recording markshaving a predetermined length. For example, DVD-RWs (Rewritable) or atype of the user data-rewritable optical recording medium employrecording marks having lengths corresponding to 3T to 11T and 14T (whereT is one clock cycle), thereby recording data.

[0006] In general, when data is recorded on an optical recording medium,the optical recording medium is not irradiated with a laser beam thathas the same pulse width as the duration corresponding to the length ofa recording mark to be formed but with a laser beam of a train of thenumber of pulses determined in accordance with the type of the recordingmark to be formed, thereby forming recording marks having apredetermined length. For example, to record data on the aforementionedDVD-RW, pulses as many as n−1 or n−2 (where n indicates the type ofrecording marks and takes on any one of 3 to 11 and 14) are successivelyimpinged thereon, thereby forming any one of the recording marks havinglengths corresponding to 3T to 11T and 14T. Accordingly, for n−2, toform a recording mark having a length corresponding to 3T, one pulse isused, while to form a recording mark having a length corresponding to11T, nine pulses are used. Furthermore, for n−1, to form a recordingmark having a length corresponding to 3T, two pulses are used, while toform a recording mark having a length corresponding to 11T, ten pulsesare used.

[0007] To form such recording marks, it is necessary to set the laserbeam to appropriate recording power for each target optical recordingmedium. However, the recording power provided by a laser beam may begreatly different from a desired level due to manufacturing variationsof the semiconductor laser emitting the laser beam, or may be out of anappropriate range due to variations caused by its service environmentsor the like. In these cases, recording marks cannot be formed in anappropriate shape, resulting in jitter value being significantlydegraded.

[0008] That is, although a countermeasure of controlling the radiationpower provided by a laser beam within the range of predetermined levelshas been taken, a problem still exists in practice that variationsresulting from the manufacturing of the semiconductor laser employed orvariations in its output power as well as its service environments maycause an excessive output power of a laser beam during operation, thusresulting in an increase in playback jitter value.

[0009] On the other hand, in recent years, there is an increasing demandfor further improved data transfer rates of the optical recordingmedium. To meet this demand, the linear speed of recording/reproductionoperations can be effectively increased, for which it is necessary toincrease clock frequencies. However, since the laser driver for drivingthe semiconductor laser is limited in its operating speed, shorteningone clock cycle (T) by increasing the clock frequency would raise aphenomenon that the pulses of the laser beam are limited in amplitude,thereby causing the next pulse to arrive before the power of the laserbeam has been lowered to a sufficient level. Such a phenomenon wouldprovide the same condition as setting the laser beam to a higherrecording power, thereby resulting in significant degradation in jittervalue.

[0010] In this context, it is desirable to form recording marks in anappropriate shape even in the aforementioned cases to provide goodjitter.

[0011] The present invention was developed in view of the aforementionedconventional problems. It is therefore an object of the presentinvention to provide an optical recording medium which can minimize anincrease in playback jitter value even in the presence of considerablevariations in the power of a laser beam employed, caused by variationsresulting from the manufacturing of a semiconductor laser emitting thelaser beam or variations in output power of the laser beam.

[0012] It is also another object of the present invention to provide anoptical recording medium which has reduced degradation in jitter whendata is recorded at high data transfer rates.

DISCLOSURE OF THE INVENTION

[0013] As a result of intensive studies, the inventor found thatdegradation in jitter value could be prevented by making the relationbetween the upper and lower limits of the recording power provided by alaser beam within a certain range. Also found was the fact that a heatsink layer provided on the side of a recording layer upon which laserbeams were incident would dissipate heat that was generated in therecording layer during irradiation with the laser beam, therebyminimizing an increase in playback jitter value even in the presence ofsignificant variations in the power of the laser beam.

[0014] That is, the aforementioned objects are achieved by the followinginventions:

[0015] (1) An optical recording medium, having at least alight-transmitting layer covered with a recording layer formed on asupport substrate, for recording information on the recording layer by alaser beam incident from the light-transmitting layer. The opticalrecording medium is characterized in that the recording layer isprovided with a heat sink layer on its light-transmitting layer side,the heat sink layer being made of a material having a thermalconductivity of greater than 1 W·m⁻¹·K⁻¹.

[0016] (2) The optical recording medium according to (1), wherein theheat sink layer is set so that Pmax>Pmin×2.2, where Pmax and Pmin arethe maximum and minimum limit powers of a laser beam having a wavelengthof 450 nm or less, respectively, at which a playback jitter value isless than 13% when reproducing information that has been recorded ontothe recording layer by being irradiated with the laser beam via aradiation optical system with an objective lens having a numericalaperture of 0.7 or more.

[0017] (3) The optical recording medium according to (2), wherein thelaser beam has a wavelength of 380 nm or more. (4) The optical recordingmedium according to (1), (2), or (3), wherein the heat sink layer has athickness of 10 nm or more and 200 nm or less, preferably 30 nm or moreand 100 nm or less.

[0018] As used herein, the term “jitter” refers to the clock jitterhaving a value that is determined as in σ/Tw (%), where σ is the signalfluctuation obtained by measuring a playback signal with a time intervalanalyzer and Tw is the detection window width.

[0019] (5) An optical recording medium, having at least a recordinglayer, for recording information by forming a recording mark on therecording layer with a laser beam, wherein a condition ofPw(max1)/Pw(min1)>2.2 is satisfied, where Pw(min1) is a lower limit ofrecording power provided by the laser beam when a jitter value of therecording mark formed is less than 13%, and Pw(max1) is an upper limitof recording power provided by the laser beam when the jitter value ofthe recording mark formed is less than 13%.

[0020] The invention according to (5) provides a wide recording powermargin for effectively reducing the occurrence of read error. This makesit possible to reduce the jitter value even when the recording laserbeam provides power at a level significantly different from a desiredone due to manufacturing variations of the semiconductor laser emittingthe laser beam, or even when the power of the laser beam is greatlyvaried due to variations of its service environments or the like. Thismakes it possible to record data with stability.

[0021] (6) The optical recording medium according to (5), wherein acondition of Pw(max1)/Pw(min1)>2.5 is satisfied.

[0022] The invention according to (6) provides a wider recording powermargin for effectively reducing the occurrence of read error, therebymaking it possible to record data with improved stability.

[0023] (7) The optical recording medium according to (5), wherein acondition of Pw(max1)/Pw(min1)>3 is satisfied.

[0024] The invention according to (7) provides a much wider recordingpower margin for effectively reducing the occurrence of read error,thereby making it possible to record data with more improved stability.

[0025] (8) The optical recording medium according to any one of (5) to(7), wherein a condition of Pw(max2)/Pw(min2)>1.5 is satisfied, wherePw(min2) is a lower limit of recording power provided by the laser beamwhen the jitter value of the recording mark formed is less than 9%, andPw(max2) is an upper limit of recording power provided by the laser beamwhen the jitter value of the recording mark formed is less than 9%.

[0026] The invention according to (8) provides a wide recording powermargin for more effectively reducing the occurrence of read error,thereby making it possible to record data with stability.

[0027] (9) The optical recording medium according to (8), wherein acondition of Pw(max2)/Pw(min2)>1.8 is satisfied.

[0028] The invention according to (9) provides a wider recording powermargin for more effectively reducing the occurrence of read error,thereby making it possible to record data with improved stability.

[0029] (10) The optical recording medium according to (9), wherein acondition of Pw(max2)/Pw(min2)>2 is satisfied.

[0030] The invention according to (10) provides a much wider recordingpower margin for more effectively reducing the occurrence of read error,thereby making it possible to record data with much improved stability.

[0031] (11) The optical recording medium according to any one of (5) to(10), further including a light-transmitting layer provided on a side ofincidence of the laser beam, and a dielectric layer and a heat sinklayer provided between the recording layer and the light-transmittinglayer.

[0032] (12) The optical recording medium according to (11), wherein theheat sink layer has a thickness of 10 to 200 nm.

[0033] The invention according to (12) can provide a wide power marginwith stability without excessively reducing the throughput of amanufacturing process.

[0034] (13) The optical recording medium according to (12), wherein theheat sink layer has a thickness of 30 to 100 nm.

[0035] The invention according to (13) can provide a wide power marginwith improved stability without excessively reducing the throughput of amanufacturing process.

[0036] (14) An optical recording medium, having at least a recordinglayer, for recording information by forming a recording mark on therecording layer with a laser beam, characterized in that a condition ofPw(max1)/Pw(min1)>2.2 is satisfied, with the laser beam having awavelength of 450 nm or less and an objective lens for focusing thelaser beam being designed to have a numerical aperture of 0.7 or more,where Pw(min1) is a lower limit of recording power provided by the laserbeam when a jitter value of the recording mark formed is less than 13%,and Pw(max1) is an upper limit of recording power provided by the laserbeam when the jitter value of the recording mark formed is less than13%.

[0037] The invention according to (14) can ensure a wide power margin toperform recording operations with stability, even in the case ofimplementing a high data transfer rate using a system for recording databy focusing a laser beam of a high energy density in a very small spot.

[0038] (15) The optical recording medium according to (14), wherein acondition of Pw(max1)/Pw(min1)>2.2 is satisfied with the laser beambeing set at a wavelength of 380 to 450 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039]FIG. 1 is a schematic cross-sectional view illustrating thestructure of layers in an optical recording medium according to anembodiment of the present invention;

[0040]FIG. 2 is a flowchart showing a method for fabricating an opticalrecording medium 10;

[0041]FIG. 3 is a schematic view illustrating the main portion of aninformation recording apparatus suitable for recording data onto theoptical recording medium 10;

[0042]FIG. 4 is a view illustrating an exemplary recording strategy forforming a recording mark of a length corresponding to 2T;

[0043]FIG. 5 is a diagram illustrating the relation between the power ofa recording laser beam and the playback jitter value for an opticalrecording medium according to Example 1 of the present invention, inconjunction with a comparative example;

[0044]FIG. 6 is a diagram illustrating the relation between the power ofa recording laser beam and the playback jitter value for an opticalrecording medium according to Example 2 of the present invention, inconjunction with Example 1 and a comparative example;

[0045]FIG. 7 is a diagram illustrating the power of a recording laserbeam and the playback jitter value for an optical recording mediumaccording to Example 3 of the present invention;

[0046]FIG. 8 is a diagram illustrating the relation between thethickness of a heat sink layer and Pmax/Pmin from the results of FIG. 7;

[0047]FIG. 9 is a graph showing the results of measurements in Example4; and

[0048]FIG. 10 is a graph showing the results of measurements in Example5.

BEST MODE FOR CARRYING OUT THE INVENTION

[0049] Now, the present invention will be explained below in more detailwith reference to the drawings in accordance with the preferredembodiments.

[0050] As shown in FIG. 1, an optical recording medium 10 according tothis embodiment includes at least a reflective film 16, a seconddielectric layer 18, a recording layer 20, a first dielectric layer 22,a heat sink layer 24, and a light-transmitting layer 26, which areformed in that order on a support substrate 12 made of polycarbonate.

[0051] In this embodiment, the support substrate 12 is formed ofpolycarbonate resin by injection molding in a thickness of about 1.1 mm.On top thereof, the reflective film 16, the second dielectric layer 18,the recording layer 20, the first dielectric layer 22, and the heat sinklayer 24 are formed in that order by sputtering, with thelight-transmitting layer 26 being formed of acrylic-based resin in athickness of about 100 μm. There is provided a hole 30 at the centerportion of the optical recording medium 10. The optical recording medium10 having such a structure is irradiated with a laser beam from thelight-transmitting layer 26 side to thereby record data, while beingirradiated with a laser beam from the light-transmitting layer 26 sideto thereby reproduce data.

[0052] Accordingly, the light-transmitting layer 26 is formed to beconsiderably thicker in thickness than a resin layer corresponding tothe position of the light-transmitting layer 26 in the optical recordingmedium 10 or a protective layer (about 5 to 10 μm in thickness) on thereflective layer in conventional CDs or DVDs or the like.

[0053] The reflective film 16 can be formed of any type of metalmaterials without limitation as long as it satisfies the requiredreflectivity, being formed of an alloy composed mainly of Ag in thisembodiment. The first dielectric layer 22 can also be formed of any typeof materials, being formed of ZnS—SiO₂ in this embodiment. The recordinglayer 20 is formed of AgInSbTeGe-based material having a phase changerecording-layer composition.

[0054] The second dielectric layer 18 is formed of a material having athermal conductivity k, where k>1 W·m⁻¹·K⁻¹, e.g., alumina (Al₂O₃), andhas a thickness of 2 nm or more and 200 nm or less, preferably, 5 nm ormore and 100 nm or less. On the other hand, the reflective film 16 has athickness of 10 to 300 nm, the recording layer 20 has a thickness of 5to 30 nm, the first dielectric layer 22 has a thickness of 10 to 300 nm,and the light-transmitting layer 26 has a thickness of 10 to 300 nm,preferably, 50 to 150 nm. However, the present invention is not limitedthereto.

[0055] The heat sink layer 24 is formed of a material having a thermalconductivity k, where k>1 W·m⁻¹·K⁻¹, e.g., alumina (Al₂O₃), and has athickness of 10 nm or more and 200 nm or less, preferably, 30 nm or moreand 100 nm or less.

[0056] The heat sink layer 24 is a layer for effectively dissipatingheat given to the recording layer 20, serving to provide an enlargedpower margin to the optical recording medium 10. Accordingly, thethermal conductivity of the heat sink layer 26 is required to be higherat least than that of the first dielectric layer 22.

[0057] More specifically, the heat sink layer 24 is set so thatPmax>Pmin×2.2, where Pmax and Pmin are the maximum and minimum limitpower of a laser beam, respectively, at which playback jitter value isless than 13% when reproducing information that has been recorded ontothe recording layer by being irradiated from the light-transmittinglayer 26 side with the laser beam having a wavelength of 450 nm or less,preferably 380 nm to 450 nm, more preferably 405 nm, via a radiationoptical system with an objective lens (not shown) having a numericalaperture of 0.7 or more, preferably 0.85.

[0058] The second dielectric layer 18 has a thickness of 2 nm or morebecause a thickness less than 2 nm would cause a significant variationin the power margin of the laser beam due to a slight variation inthickness. Thus, more preferably, the aforementioned thickness is to be5 nm or more.

[0059] The heat sink layer 24 has a thickness of 10 nm or more becausethe heat sink layer 24 being specifically less than 10 nm in thicknesswould make its thickness control difficult while allowing the powermargin of the laser beam to be significantly varied due to a slightvariation in thickness in the same manner as described above. Thethickness being 30 nm or more would make it possible to provide anoticeably increased power margin. Accordingly, the thickness ispreferably determined to be 30 nm or more.

[0060] Furthermore, the aforementioned thickness is 200 nm or lessbecause a thickness more than 200 nm would require an excessive time forits deposition during manufacturing, thereby resulting in reducedthroughput as well as increased thermal damage to the support substrate12. Accordingly, the thickness is determined to be 200 nm or less, morepreferably 100 nm or less. Taking the foregoing into consideration, thethickness of the heat sink layer 24 is preferably set at 10 to 200 nm,more preferably at 30 to 100 nm.

[0061] In the aforementioned embodiment, the second dielectric layer 18and the heat sink layer 24 are made of alumina. The present invention isnot limited thereto but may also employ a material having a thermalconductivity within the aforementioned range and formable in the shapeof film for covering the recording layer 20 therewith, e.g., aluminumnitride or the like.

[0062] Additionally, in the aforementioned embodiment, thelight-transmitting layer 26 is made of an acryl-based resin; however,any type of material can also be selected from the group of an energybeam curable resin that is hardened by an energy beam such as anultraviolet ray or a thermally curable resin that is hardened by heat,thus making the acryl-based resin, the epoxy-based resin, theurethane-based resin or the like applicable. It is also possible toemploy a pre-formed resin sheet such as of polycarbonate or polyolefin.

[0063] Furthermore, the support substrate 12 may also be made ofpolyolefin or the like other than the polycarbonate as employed in theembodiment.

[0064] The recording layer 20 (AgInSbTeGe) of the optical recordingmedium 10 is made of a phase change film having different values ofreflectivity between in the crystalline state and in the amorphousstate, which is utilized for recording data. More specifically, therecording layer 20 of a non-recorded area is in a crystalline state with20% reflectivity, for example. To record any data onto such anon-recorded area, a predetermined portion in the recording layer 20 isheated to a temperature higher than the melting point in accordance withthe data to be recorded and then quickly cooled down into an amorphousstate. The portion in the amorphous state has, e.g., 7% reflectivity,thus allowing the predetermined data to be recorded. To overwrite thedata once recorded, the portion of the recording layer 20 in which thedata to be overwritten is recorded is heated to a temperature higherthan the crystallization point or the melting point in accordance withthe data to be recorded and thus changed into the crystalline state oran amorphous state.

[0065] In this case, the relation between the power Pw (recording power)of a laser beam with which the recording layer 20 is irradiated to melt,the power Pb (ground power) of a laser beam with which the recordinglayer 20 is irradiated to cool down, and the Pe (erasing power) of alaser beam with which the recording layer 20 is irradiated tocrystallize is expressed by Pw>Pe≧Pb.

[0066] The optical recording medium 10 according to this embodimentpreferably stores “recording condition setting information,” though thepresent invention is not limited thereto. The recording conditionsetting information refers to various conditions necessary torecord/reproduce data on/from the optical recording medium 10, e.g.,information used for identifying the power of a recording laser beam ora recording strategy. The recording condition setting informationincludes not only those pieces specifically indicative of each conditionnecessary to record/reproduce data but also those pieces for identifyinga recording/reproduction condition by specifying any of the variousconditions stored in an information recording apparatus.

[0067] On the other hand, the “recording strategy” refers to a method ofirradiation with a recording laser beam to form recording marks, i.e.,the settings such as the number of laser beam pulses, the pulse width ofeach pulse, the pulse interval, and the power of the laser beam (Pw, Pe,and Pb), which are determined in accordance with the recording conditionsetting information included in the optical recording medium 10.

[0068] To record data on the optical recording medium 10 according tothis embodiment, it is necessary to set the recording power Pw of alaser beam within an appropriate range. If the recording power Pw is outof the appropriate range, recording marks cannot be formed in a propershape, thus causing jitter to be significantly degraded. The range ofthe recording power Pw in which the jitter value of the recording markformed is 13% or less is referred to as “the first power margin,” thelower limit power of which is defined as Pw(min1) and the upper limitpower of which is defined as Pw(max1). In this case, the opticalrecording medium 10 according to this embodiment allows the ratio of theupper limit power Pw(max1) to the lower limit power Pw(min1) to satisfythe condition given by

Pw(max1)/Pw(min1)>2.2  (1).

[0069] Preferably, the ratio of the upper limit power Pw(max1) to thelower limit power Pw(min1) satisfies the condition given by

Pw(max1)/Pw(min1)>2.5  (2), and

[0070] more preferably, the condition given by

Pw(max1)/Pw(min1)>3  (3).

[0071] In general, with jitter values exceeding 13%, a large amount ofread errors would occur. Accordingly, information necessary to set therecording power Pw of a laser beam within the range of Pw(min1) toPw(max1) is contained in the recording condition setting information,thereby making it possible to effectively reduce the occurrence of readerrors.

[0072] The range of the recording power Pw in which the jitter value ofthe recording mark formed is 9% or less is referred to as “the secondpower margin,” the lower limit power of which is defined as Pw(min2) andthe upper limit power of which is defined as Pw(max2). In this case, theoptical recording medium 10 according to this embodiment allows theratio of the upper limit power Pw(max2) to the lower limit powerPw(min2) to satisfy the condition given by

Pw(max2)/Pw(min2)>1.5  (4),

[0073] preferably, the condition given by

Pw(max2)/Pw(min2)>1.8  (5), and

[0074] more preferably, the condition given by

Pw(max2)/Pw(min2)>2  (6).

[0075] In general, with jitter values exceeding 9%, there would be ahigh possibility of occurrence of read errors. Accordingly, informationnecessary to set the recording power Pw of a laser beam within the rangeof Pw(min2) to Pw(max2) is contained in the recording condition settinginformation, thereby making it possible to effectively reduce theoccurrence of read errors.

[0076] Now, a method for fabricating the optical recording medium 10according to this embodiment will be described below.

[0077]FIG. 2 is a flowchart showing the method for fabricating theoptical recording medium 10 according to this embodiment. As describedabove, the light-transmitting layer 26 of the optical recording medium10 is as very thin as 10 to 300 μm in thickness, and is thereforedeposited in the reverse order to that of the conventional generalDVD-RW.

[0078] First, a stamper is used to injection mold a support substrate 12having a thickness of about 1.1 mm, with a pre-groove of groove widthabout 0.151 μm, track pitch about 0.32 μm, and groove depth about 20 nm(step S1).

[0079] Then, the support substrate 12 is transported into a firstchamber (not shown) of a sputtering apparatus. The sputtering apparatusis provided in the first chamber with an alloy composed mainly of silveras a target. Then, the first chamber is pumped into a vacuum of about1×10⁴ Pa. Subsequently, an argon gas is introduced into the firstchamber to set the gas pressure at 0.1 to 1.0 Pa. Thereafter, a DC or RFvoltage is applied to the target for sputtering. In this manner, on topof the support substrate 12, there is formed a reflective film 16 of 10to 300 nm in thickness (step S2).

[0080] Then, the support substrate 12 having the reflective film 16formed thereon is transported from the first chamber to a second chamber(not shown). In the second chamber of the sputtering apparatus, providedis Al₂O₃ as a target.

[0081] Then, the second chamber is pumped into a vacuum of about 1×10⁻⁴Pa. Subsequently, an argon gas is introduced into the second chamber toset the gas pressure at 0.1 to 1.0 Pa for sputtering. In this manner, ontop of the reflective film 16, there is formed a second dielectric layer18 having a thickness of 2 to 50 nm (step S3).

[0082] Then, the support substrate having the reflective film 16 and thesecond dielectric layer 18 formed thereon is transported from the secondchamber to a third chamber (not shown). In the third chamber of thesputtering apparatus, provided is a target mixture of Ag, In, Sb, Te,and Ge. Then, the third chamber is pumped into a vacuum of about 1×10⁻⁴Pa. Subsequently, an argon gas is introduced into the third chamber toset the gas pressure at 0.1 to 1.0 Pa for sputtering. In this manner, ontop of the second dielectric layer 18, there is formed a recording layer20 having a thickness of 5 to 30 nm (step S4).

[0083] Then, the support substrate 12 having the reflective film 16 tothe recording layer 20 formed thereon is transported from the thirdchamber to a fourth chamber (not shown). In the fourth chamber of thesputtering apparatus, provided is a target mixture of ZnS and SiO₂.Then, the fourth chamber is pumped into a vacuum of about 1×10⁻⁴ Pa.Subsequently, an argon gas is introduced into the fourth chamber to setthe gas pressure at 0.1 to 1.0 Pa for sputtering. In this manner, on topof the recording layer 20, there is formed a first dielectric layer 22having a thickness of 10 to 300 nm (step S5).

[0084] Then, the support substrate 12 having the reflective film 16 tothe first dielectric layer 22 formed thereon is transported from thefourth chamber to a fifth chamber (not shown). In the fifth chamber ofthe sputtering apparatus, provided is a target of Al₂O₃. Then, the fifthchamber is pumped into a vacuum of about 1×10⁻⁴ Pa. Subsequently, anargon gas is introduced into the fifth chamber to set the gas pressureat 0.1 to 1.0 Pa for sputtering. In this manner, on top of the firstdielectric layer 22, there is formed a heat sink layer 24 having athickness of 10 to 200 nm, preferably 30 to 100 nm (step S6). Thesputtering of the heat sink layer 24 may also be carried out in thesecond chamber.

[0085] In this manner, the reflective film 16, the second dielectriclayer 18, the recording layer 20, the first dielectric layer 22, and theheat sink layer 24 are formed on the support substrate. Then, asubstrate 11 having each of these layers formed thereon is taken out ofthe fifth chamber of the sputtering apparatus and then coated on thesurface of the heat sink layer 24 with an UV curable resin such as byspin coating, by roll coating, or by screen printing. Then, theresulting substrate 11 is irradiated with an ultraviolet radiation tothereby form a light-transmitting layer 26 having a thickness of about10 to 300 μm (step S7). In this manner, the optical recording medium 10according to this embodiment is completed. To form thelight-transmitting layer 26, a pre-molded resin sheet material such asof polycarbonate or polyolefin may also be adhered to the surface of theheat sink layer 24.

[0086] Now, an apparatus for recording data onto the optical recordingmedium 10 according to this embodiment will be described below.

[0087]FIG. 3 is a schematic view illustrating the main portion of aninformation recording apparatus suitable for recording data onto theoptical recording medium 10.

[0088] As shown in FIG. 3, such an information recording/reproducingapparatus includes a spindle motor 2 for rotating the optical recordingmedium 10, a head 3 for irradiating the optical recording medium 10 witha laser beam, a controller 4 for controlling the operations of thespindle motor 2 and the head 3, a laser drive circuit 5 for supplying alaser drive signal to the head 3, and a lens drive circuit 6 forsupplying a lens drive signal to the head 3.

[0089] As shown in FIG. 3, the controller 4 includes a focus servofollower circuit 7, a tracking servo follower circuit 8, and a lasercontrol circuit 9. Activating the focus servo follower circuit 7 wouldallow the recording surface of the optical recording medium 10 beingrotated to be focused, while activating the tracking servo followercircuit 8 would allow the spot of a laser beam to automatically followan eccentric signal track of the optical recording medium 10. The focusservo follower circuit 7 and the tracking servo follower circuit 8 areprovided with an automatic gain control function for automaticallyadjusting focus gain and an automatic gain control function forautomatically adjusting tracking gain, respectively. The laser controlcircuit 9 generates a laser drive signal to be supplied by the laserdrive circuit 5, while generating an appropriate laser drive signal inaccordance with recording condition setting information stored in theoptical recording medium 10, if any.

[0090] These focus servo follower circuit 7, the tracking servo followercircuit 8, and the laser control circuit 9 do not always need to beincorporated into the controller 4 but may also be prepared separatelyfrom the controller 4. Furthermore, these circuits need not to be alwaysin the form of a physical circuit but may also be in the form ofsoftware to be executed in the controller 4.

[0091] Although not limited to the following particular arrangement, theinformation recording apparatus suitable for recording data onto theoptical recording medium 10 employs preferably a laser beam ofwavelength 450 nm or less, particularly 380 to 450 nm, and an objectivelens or part of the head 3 for focusing a recording laser beam, havingan NA (numerical aperture) of 0.7 or more. In recording data onto theoptical recording medium 10 using such an information recordingapparatus, a distance (working distance) to be set between the objectivelens and the surface of the optical recording medium 10 is very small(e.g., about 80 to 150 μm), thereby making it possible to realize a beamspot of a significantly reduced diameter as compared with theconventional one. This makes it possible to realize an extremely highdata transfer rate (e.g., 35 Mbps or greater) in recording data onto theoptical recording medium 10 using such an information recordingapparatus.

[0092] Additionally, as described above, in recording data onto theoptical recording medium 10 according to this embodiment using such aninformation recording apparatus, a recording strategy determined inaccordance with recording condition setting information stored on theoptical recording medium 10, if any, is used to determine the recordingpower Pw of the laser beam.

[0093] The aforementioned information recording apparatus 30 can alsoemploy a (1, 7) RLL modulation scheme, though the present invention isnot limited thereto. However, the information recording apparatus forrecording data onto the optical recording medium 10 does not always needto employ such a modulation scheme to record data but may also employother modulation schemes for recording data.

[0094] Now, an exemplary recording strategy will be described belowwhich employs the (1, 7) RLL modulation scheme.

[0095]FIG. 4 is a view illustrating an exemplary recording strategy forforming a recording mark of a length corresponding to 2T.

[0096] As shown in FIG. 4, to form a recording mark of a lengthcorresponding to 2T, the number of laser beam pulses is set at “1.” Inthe foregoing, the number of laser beam pulses is defined by the numberof times of raising the power of the laser beam up to Pw. In moredetail, suppose that the timing at which the laser beam is positioned atthe start point of a recording mark is time ts, and the timing at whichthe laser beam is positioned at the end point of the recording mark istime te. In this case, the power of the laser beam is raised once up toPw and then to power Pb during the period from time ts to time te. Thepower of the recording laser beam is set at Pe before time ts, allowingthe laser beam to rise at time ts. On the other hand, the power of thelaser beam is set at Pe or Pb at time te.

[0097] During the duration of Tpulse, the recording layer 20 of theoptical recording medium 10 is subjected to a high energy to have atemperature higher than the melting point, whereas during the durationof Tcl, the recording layer 20 of the optical recording medium 10 isquickly cooled down. This allows the recording mark of a lengthcorresponding to 2T to be formed in the recording layer 20 of theoptical recording medium 10.

[0098] To form a recording mark of another length, like the recordingmark of a length corresponding to 2T being formed, the power of thelaser beam is set at Pw, Pe, or Pb, thus forming recording marks havinga predetermined length each with a predetermined number of pulses.

[0099] The optical recording medium 10 according to this embodimentprovides a wide power margin as described above. It is thereforepossible to reduce jitter even when the power of the laser beam issignificantly different from the desired level due to manufacturingvariations of the laser driver included in the head 3 or in the presenceof variations in the power of the laser beam caused for some reason. Inparticular, the optical recording medium 10 according to this embodimentcan provide good jitter even when a laser beam of a relatively strongrecording power is used to form recording marks. Accordingly, asufficient power margin can be secured even when recording is carriedout at a high data transfer rate setting (e.g., 35 Mbps or more).

[0100] Now, examples of the present invention will be explained indetail below.

EXAMPLE 1

[0101] An optical recording medium was prepared in accordance with thefollowing procedures.

[0102] A disc-shaped support substrate was employed which was made of apolycarbonate resin like in the aforementioned embodiment and which hada surface having grooves formed thereon (the depth of the grooves wasλ/18 when expressed by an optical path length at a wavelength λ=405 nmwith a record track pitch of 0.32 μm). On this surface, formed was areflective film composed mainly of Ag by sputtering in a thickness of100 nm.

[0103] Then, on the surface of the reflective film, formed was a seconddielectric layer of Al₂O₃ by sputtering in a thickness of 20 nm.

[0104] Additionally, a recording layer was formed by sputtering in athickness of 12 nm on the second dielectric layer using an alloy targetof a phase change material. This recording layer was chosen to have acomposition of AgInSbTeGe.

[0105] Furthermore, on the surface of the recording layer, a firstdielectric layer was formed by sputtering in a thickness of 45 nm usinga ZnS(80 mol %)-SiO₂ (20 mol %) target.

[0106] Like the aforementioned second dielectric layer, a heat sinklayer of Al₂O₃ was formed by sputtering in a thickness of 30 nm on thesurface of the first dielectric layer.

[0107] Then, an UV curable resin was coated by spin coating on thesurface of the heat sink layer, and then irradiated with an ultravioletradiation to thereby obtain a light-transmitting layer having athickness of 100 μm.

COMPARATIVE EXAMPLE 1 COMPARATIVE EXAMPLE 2

[0108] Furthermore, the heat sink layer was removed from theaforementioned example 1 to prepare comparative example 1, while thesecond dielectric layer of the aforementioned comparative example 1 wassubstituted into the same dielectric layer as the first dielectric layerof the aforementioned example 1 and the comparative example 1 to preparecomparative example 2.

[0109] As with the aforementioned embodiment, recording was performedwith these comparative examples under the conditions of a wavelengthλ=405 nm and a numerical aperture of the objective lens NA=0.85 with aratio of Pe/Pw fixed to 0.55 (where Pe is the erasing power and Pw isthe recording power) and with the power of the laser beam varied in arange of 3 mW to 10 mW. Thereafter, the playback jitter value wasmeasured as shown in FIG. 5. In the foregoing, the recording wasperformed only on one track (groove).

[0110] With reference to FIG. 5, Table 1 shows in comparison the maximumlimit power Pmax and the minimum limit power Pmin at which the playbackjitter value was below 13% in the aforementioned example 1, comparativeexample 1, and comparative example 2. TABLE 1 Pmin Pmax Pmax/PminExample 1 4.2 Unmeasurable >2.2 Comparative 4.2 8.5 2.02 example 1Comparative 2.8 5.9 2.11 example 2

[0111] As can be seen from FIG. 5 and Table 1 as well, in thecomparative examples 1 and 2, the maximum limit power Pmax is abouttwice the minimum limit power Pmin of the laser beam at which theplayback jitter value is below 13%. However, in the example 1, as canalso be seen from FIG. 2, the maximum limit power exceeds 10 mW and isthus unmeasurable while the minimum limit power of the laser beam atwhich the playback jitter value is below 13% is 4.2 mW.

[0112] That is, in the example 1, Pmax was more than 2.2 times Pmin.

EXAMPLE 2

[0113] The example 2 is provided with a high recording sensitivity bychanging the first dielectric layer of the example 1 to 35 nm inthickness (assuming the example 1 has a low sensitivity).

[0114] Measurements were made on the playback jitter values of theserecording media having a low and high sensitivity and the recordingmedium according to the comparative example 2 to obtain the values asshown in FIG. 6.

[0115] Like Table 1 above, Table 2 shows the minimum limit power Pminand the maximum limit power Pmax of the laser beam according to theexamples of a low and high sensitivity and the comparative example 2,and Pmax/Pmin. TABLE 2 Pmin Pmax Pmax/Pmin Example 1 4.2 >10 >2.2 (Lowsensitivity) Example 2 3.2 10.1 3.16 (High sensitivity) Comparative 2.85.9 2.11 example 2

[0116] As can be seen from FIG. 6 and Table 2 as well, the example ofthe present invention provides Pmax/Pmin greater than or equal to 2.2.

EXAMPLE 3

[0117] Now, the heat sink layer of the optical medium according to theexample 2 was changed in thickness to measure how it would affect theplayback jitter value, the measurements being obtained as shown in FIG.7.

[0118] The heat sink layer was formed in a thickness of three types ofvalues 10 nm, 30 nm, and 60 nm. FIG. 7 also shows a comparative exampleof the heat sink layer having a thickness of 0 nm.

[0119]FIG. 8 illustrates the relation between the Pmax/Pmin and thethickness of the heat sink layer according to these examples.

[0120] From FIGS. 7 and 8, it can be seen that the heat sink layerhaving even an slight thickness provides Pmax/Pmin greater than 2.2.Additionally, FIG. 8 shows that the heat sink layer having a thicknessof even about 100 nm can provide Pmax/Pmin greater than or equal to 2.2.It can also be expected that a thickness of even about 200 nm wouldprovide Pmax/Pmin greater than 2.2.

EXAMPLE 4

[0121] The aforementioned method was employed to fabricate opticalrecording media 10-1, 10-2, 10-3, and 10-4 in the structure shown inFIG. 1 each with the support substrate 12 having a thickness of 1.1 mm,the reflective film 16 having a thickness of 100 nm, the seconddielectric layer 18 having a thickness of 20 nm, the recording layer 20having a thickness of 12 nm, the first dielectric layer 15 having athickness of 35 nm, the light-transmitting layer 26 having a thicknessof 100 μm, and the heat sink layer 24 having respective thicknesses of 0nm, 10 nm, 30 nm, and 60 nm.

[0122] On these optical recording media 10-1, 10-2, 10-3, and 10-4,various types of recording power Pw (10 mW at maximum) were employed tocreate a signal mixture made up of recording marks having lengthscorresponding to 2T to 8T under the conditions shown in Table 3. The (1,7) RLL modulation scheme was employed for recording data only on onetrack. TABLE 3 Clock frequency 66 MHz Clock cycle (1T) 15.15 nsec Linearspeed 5.3 m/sec Modulation scheme (1, 7)RLL Format efficiency 80% Datatransfer rate 35 Mbps (efficiency considered) Channel bit length 0.12μm/bit Numerical aperture (NA) 0.85 Laser wavelength 405 nm Pe Pw × 0.55Pb 0.1 mW

[0123] Then, the clock jitter of the signal mixture created on theoptical recording media 10-1, 10-2, 10-3, and 10-4 was measured. In themeasurements, a time interval analyzer was used to determine the“fluctuation σ” of the playback signal to calculate σ/Tw (where Tw isone clock cycle).

[0124] The results of the measurements are shown in FIG. 9. The legendsindicated in parentheses of those shown in FIG. 9 are the thicknesses ofthe heat sink layer 16.

[0125] As shown in FIG. 9, it can be seen that the optical recordingmedia 10-2, 10-3, and 10-4 having the heat sink layer 24 provide a muchwider power margin than the optical recording medium 10-1 having no heatsink layer 24 (0 nm). Table 4 shows specific power margins. TABLE 4Pw(max1)/ Pw(max2)/ Pw(min1) Pw(min2) Pw(max2) Pw(max1) Pw(min1)Pw(min2) Optical recording 4.7 mW 5.7 mW 5.9 mW  9.3 mW 1.98 1.04 medium10-1 Optical recording 2.8 mW 3.2 mW 6.4 mW  8.4 mW 3.00 2.00 medium10-2 Optical recording 3.2 mW 3.7 mW 6.6 mW >10 mW >3.13 1.78 medium10-3 Optical recording 3.0 mW 3.6 mW 7.1 mW >10 mW >3.33 1.97 medium10-4

[0126] As shown in Table 4, it is seen that the optical recording medium10-1 having no heat sink layer 24 (0 nm) provides Pw(max1)/Pw(min1)<2.2,whereas the optical recording media 10-2, 10-3, and 10-4 having the heatsink layer 16 provide Pw(max1)/Pw(min1)>2.2. In particular, the opticalrecording media 10-3 and 10-4 with the heat sink layer 24 having athickness of 30 nm and 60 nm, respectively, produce jitter below 1.3% ata recording power setting of 10 mW, with Pw(max1)/Pw(min1)>3. Asdescribed above, it is seen that the optical recording media 10-3 and10-4 with the heat sink layer 24 having a thickness of 30 nm and 60 nm,respectively, have a region of jitter 13% or less, i.e., a very widefirst power margin.

[0127] Furthermore, as shown in Table 4, the optical recording medium10-1 having no heat sink layer 24 (0 nm) provides Pw(max2)/Pw(min2)<1.5,whereas the optical recording media 10-2, 10-3, and 10-4 having the heatsink layer 24 provide Pw(max2)/Pw(min2)>1.5. In particular, the opticalrecording media 10-2 and 10-4 with the heat sink layer 24 having athickness of 10 nm and 60 nm, respectively, providePw(max2)/Pw(min2)>1.8. As described above, it is seen that the opticalrecording media 10-2 and 10-4 with the heat sink layer 24 having athickness of 30 nm and 60 nm, respectively, have a region of jitter 9%or less, i.e., a very wide second power margin.

[0128] From this example, it was confirmed that setting the heat sinklayer 24 to a thickness of 10 nm or greater, with the other layershaving the same structure, will provide an enlarged power margin, and inparticular, setting the heat sink layer 24 at 60 nm in thickness willprovide a significantly enlarged power margin.

EXAMPLE 5

[0129] The aforementioned method was employed to fabricate an opticalrecording medium 10-5 in the structure shown in FIG. 1, with the supportsubstrate having a thickness of 1.1 mm, the reflective film 16 having athickness of 100 nm, the second dielectric layer 18 having a thicknessof 20 nm, the recording layer 14 having a thickness of 12 nm, the firstdielectric layer 22 having a thickness of 45 nm, and the heat sink layer24 having a thickness of 30 nm. This optical recording medium 10-5 isdifferent from the aforementioned optical recording medium 10-3 only inthat the first dielectric layer 22 has been changed in thickness. Thiscauses the optical recording medium 10-5 to have a lower recordingsensitivity than the aforementioned optical recording medium 10-3 does.

[0130] On the optical recording medium 10-5, various types of recordingpower Pw (10 mW at maximum) were employed to create a signal mixturemade up of recording marks having lengths corresponding to 2T to 8Tunder the conditions shown in Table 3. The (1, 7) RLL modulation schemewas employed for recording data on only one track.

[0131] Then, the clock jitter of the signal mixture created on theoptical recording medium 10-5 was measured.

[0132] The results of the measurements are shown in FIG. 10. FIG. 10also shows the results of the measurements made on the optical recordingmedium 10-3 with the heat sink layer 24 having the same thickness asthat in the optical recording medium 10-5. The legends indicated inparentheses of those shown in FIG. 6 are the thicknesses of the firstdielectric layer 22.

[0133] As described above, it can be confirmed that the opticalrecording medium 10-5 has a lower recording sensitivity than the opticalrecording medium 10-3 does, so that the sufficiently low jitter regionis shifted toward the higher output power, but provides a sufficientlywide power margin like the optical recording medium 10-3. Table 5 showsspecific power margins. TABLE 5 Pw(max1)/ Pw(max2)/ Pw(min1) Pw(min2)Pw(max2) Pw(max1) Pw(min1) Pw(min2) Optical recording 4.2 mW 4.9 mW >10mW >10 mW >2.38 >2.04 medium 10-5

[0134] As shown in Table 5, the optical recording medium 10-5 producesjitter below 13% at a recording power setting of 10 mW, withPw(max1)/Pw(min1)>2.2.

[0135] Furthermore, as shown in Table 3, the optical recording medium10-5 produces jitter below 9% at a recording power setting of 10 mW,with Pw(max2)/Pw(min2)>2.

[0136] From this example, it was confirmed that with the heat sink layer24 having the same thickness, the first dielectric layer 22 could alsobe changed in thickness to adjust the recording sensitivity, whileproviding a sufficiently wide power margin.

[0137] As described above, the optical recording medium 10 according tothis embodiment is adapted such that the ratio of the upper limit powerPw(max1) to the lower limit power Pw(min1) satisfies the condition givenby

Pw(max1)/Pw(min1)>2.2  (1),

[0138] thus providing reduced jitter even in the presence of variationsin power of the recording laser beam caused for some reason.

[0139] The present invention is not limited to the aforementionedembodiment, but various modifications can also be made thereto withinthe scope of the present invention defined by the appended claims, andthose modifications are to be included in the scope of the presentinvention.

[0140] For example, in the aforementioned embodiment, the structure ofFIG. 1 was shown as a specific structure of the optical recording medium10; however, the structure of the optical recording medium according tothe present invention is not limited thereto.

INDUSTRIAL APPLICABILITY

[0141] The present invention is configured as described above, and thushas an advantageous effect of being able to provide an enlarged powermargin to a recording laser beam. Furthermore, the optical recordingmedium according to the present invention provides a very wide powermargin, thereby making it possible to record data with stability.

1. An optical recording medium, having at least a light-transmittinglayer covered with a recording layer formed on a support substrate, forrecording information on the recording layer by a laser beam incidentfrom the light-transmitting layer, wherein the recording layer isprovided with a heat sink layer on its light-transmitting layer side,the heat sink layer being made of a material having a thermalconductivity of greater than 1 W·m⁻¹·K⁻¹.
 2. The optical recordingmedium according to claim 1, wherein the heat sink layer is set so thatPmax>Pmin×2.2, where Pmax and Pmin are maximum and minimum limit powersof a laser beam having a wavelength of 450 nm or less, respectively, atwhich a playback jitter value is less than 13% when reproducinginformation that has been recorded onto the recording layer by beingirradiated with the laser beam via a radiation optical system with anobjective lens having a numerical aperture of 0.7 or more.
 3. Theoptical recording medium according to claim 2, wherein the laser beamhas a wavelength of 380 nm or more.
 4. The optical recording mediumaccording to claim 1, 2, or 3, wherein the heat sink layer has athickness of 10 nm or more and 200 nm or less, preferably 30 nm or moreand 100 nm or less.
 5. An optical recording medium, having at least arecording layer, for recording information by forming a recording markon the recording layer with a laser beam, wherein a condition ofPw(max1)/Pw(min1)>2.2 is satisfied, where Pw(min1) is a lower limit ofrecording power provided by the laser beam when a jitter value of therecording mark formed is less than 13%, and Pw(max1) is an upper limitof recording power provided by the laser beam when the jitter value ofthe recording mark formed is less than 13%.
 6. The optical recordingmedium according to claim 5, wherein a condition ofPw(max1)/Pw(min1)>2.5 is satisfied.
 7. The optical recording mediumaccording to claim 5, wherein a condition of Pw(max1)/Pw(min1)>3 issatisfied.
 8. The optical recording medium according to any one ofclaims 5 to 7, wherein a condition of Pw(max2)/Pw(min2)>1.5 issatisfied, where Pw(min2) is a lower limit of recording power providedby the laser beam when the jitter value of the recording mark formed isless than 9%, and Pw(max2) is an upper limit of recording power providedby the laser beam when the jitter value of the recording mark formed isless than 9%.
 9. The optical recording medium according to claim 8,wherein a condition of Pw(max2)/Pw(min2)>1.8 is satisfied.
 10. Theoptical recording medium according to claim 9, wherein a condition ofPw(max2)/Pw(min2)>2 is satisfied.
 11. The optical recording mediumaccording to any one of claims 5 to 10, further including alight-transmitting layer provided on a side of incidence of the laserbeam, and a dielectric layer and a heat sink layer provided between therecording layer and the light-transmitting layer.
 12. The opticalrecording medium according to claim 11, wherein the heat sink layer hasa thickness of 10 to 200 nm.
 13. The optical recording medium accordingto claim 12, wherein the heat sink layer has a thickness of 30 to 100nm.
 14. An optical recording medium, having at least a recording layer,for recording information by forming a recording mark on the recordinglayer with a laser beam, wherein a condition of Pw(max1)/Pw(min1)>2.2 issatisfied, with the laser beam having a wavelength of 450 nm or less andan objective lens for focusing the laser beam being designed to have anumerical aperture of 0.7 or more, where Pw(min1) is a lower limit ofrecording power provided by the laser beam when a jitter value of therecording mark formed is less than 13%, and Pw(max1) is an upper limitof recording power provided by the laser beam when the jitter value ofthe recording mark formed is less than 13%.
 15. The optical recordingmedium according to claim 14, wherein a condition ofPw(max1)/Pw(min1)>2.2 is satisfied with the laser beam being set at awavelength of 380 to 450 nm.